U.S. Pat. No. 6,255,730 B1 (to Dove, Casey and Blume, issued Jul. 3, 2001) describes various thick film techniques that become possible with the recent advent of certain dielectric materials. These are KQ-120 and KQ-CL907406, which are products of Heraeus Cermalloy, 24 Union Hill Road, West Conshohocken, Pa. Hereinafter, we shall refer to these products as the xe2x80x9cKQ dielectric,xe2x80x9d or as simply xe2x80x9cKQ.xe2x80x9d In particular, that Patent describes the construction of an xe2x80x9cencapsulatedxe2x80x9d microstrip transmission line. This Disclosure concerns further novel and useful thick film techniques pertaining to an encapsulated microstrip transmission line, not heretofore practical, that may be practiced with these KQ dielectric materials. Accordingly, U.S. Pat. No. 6,255,730 B1 is hereby incorporated herein by reference.
A xe2x80x9chybridxe2x80x9d circuit consisting of a substrate with various thick film structures thereon that are interconnected with a plurality of ICs continues to be an attractive technique for creating functionally complex high frequency assemblies from xe2x80x9ccomponentxe2x80x9d ICs. It is often the case that it is necessary or very desirable to use transmission lines to interconnect these ICs, or in connecting them to an external environment. We are particularly interested in the case when the transmission line is of the encapsulated microstrip type described in the incorporated Patent. By the term xe2x80x9cencapsulatedxe2x80x9d that Patent means that the transmission line, which in their example is what would otherwise be called a microstrip, is fully shielded, with a ground completely surrounding the center conductor. It is not exactly what we would ordinarily term a xe2x80x9ccoaxialxe2x80x9d transmission line, since its cross section does not exhibit symmetry about an axis; it has a line and a rectangular trapezoid for a cross section instead of a fat point and surrounding circle. Nevertheless, we shall find it appropriate and convenient to call it (the xe2x80x98encapsulatedxe2x80x99 transmission line of the ""730 B1 Patent) a xe2x80x98quasi-coaxialxe2x80x99 transmission line, which, it should be noted, is pretty small (perhaps 0.050xe2x80x3 wide by 0.010xe2x80x3 or 0.015xe2x80x3 high).
It will be convenient for the description that follows to introduce a useful terminology to describe the parts of a quasi-coaxial transmission line. We shall say that it has a bottom-half ground shield (which may be that portion of an entire ground plane that is covered by the quasi-coaxial transmission line, or be a meandering ribbon of metal along the path the quasi-coaxial transmission line is to take). Above that is a bottom-half portion or layer of KQ dielectric material, on top of which is a (narrower) metal layer serving as center conductor, followed above by a top-half layer of KQ dielectric material (preferably not quite as wide as the bottom-half layer of KQ material, but in any event, wider than the center conductor), and all of which is enclosed by a top-half ground shield whose edges contact the bottom-half ground shield. This terminology is not found in the incorporated ""730 B1 Patent, and it omits the optional loading resistances (210), but will otherwise be found to be in perfect agreement with FIG. 2 of that Patent, and with its description therein.
So now we have hybrids with quasi-coaxial transmission lines to interconnect things. We also have an old problem familiar to those who have laid out printed circuit boards: two traces need to cross each other, and no amount of laying things out another way will avoid it. The same situation can arise with our quasi-coaxial transmission lines on a substrate: we feel the need for one of them to cross over of a substrate to the other, allowing the cross-over to occur by virtue of the two paths being now on opposite sides. This is an expensive solution for many reasons (holes in ceramic are a last resort, and it may be very desirable for mounting and heat removal purposes to not have anything on one side of the substrate, etc.), not to mention that the effect on the transmission line itself would likely be extreme. The transition from one side of the substrate to the other would, in all probability, introduce a severe discontinuity. Other solutions that involve bridging with a short length of conductor, say, tiny coax, to jump one transmission line over the other, have their own significant drawbacks. What to do?
A solution to the problem of crossing quasi-coaxial transmission lines fabricated on a substrate is to first fabricate all the quasi-coaxial transmission lines that are to remain at substrate level and be xe2x80x9ccrossed.xe2x80x9d There are two cases: a true ground plane of metal will cover the substrate, or, each quasi-coaxial transmission line will have its own separate meandering bottom-half ground shield. In either case, when the crossed quasi-coaxial transmission lines are complete they will have top-half ground shields connected to metal against the substrate that is ground. In the case of a true ground plane, it is now looks as if it has an infestation of worms. If there is no ground plane the xe2x80x9ccrossingxe2x80x9d quasi-coaxial transmission line that is to cross over must now have its bottom-half ground shield applied. It can overlay any top-half ground shield for any crossed quasi-coaxial transmission line that is in its path. If there is a ground plane, then that step is not necessary, since ground is everywhere, already. Now a bottom-half layer of KQ dielectric material is applied along the path of the crossing quasi-coaxial transmission line, including right over the top of any crossed quasi-coaxial transmission line. To this bottom-half layer of KQ dielectric material is applied a layer of metal that becomes the center conductor of the crossing quasi-coaxial transmission line. Subsequently, the crossing quasi-coaxial transmission line is finished by printing a top-half layer of KQ dielectric, covered by a layer of metal that is the top-half of ground shield. The edges of this top-half ground shield touch either the ground plane or the outer portion of the bottom-half ground shield, which was made wide enough for that purpose.